The present invention relates to refiners, and more particularly, to the pattern of bars and grooves on the rotating plates of disc refiners.
Disc refiners are generally classified into "low consistency" and "high consistency" types. The former operate at ambient pressure on a feed slurry having a solids content of up to about 15%, whereas the latter operate on feed material having a solids content above about 30%, delivered under pressure. These percentages refer to the refining of lignocellulosic material, but the use of rotating disc refiners is not limited thereto.
Although low consistency and high consistency refiners operate in different regimes, the basic structure of the refiner is similar in one important respect. The refining action occurs as feed material passes generally radially outwardly between opposed, substantially circular, relatively rotating plates having a pattern of bars and intervening grooves. Work is performed on the feed material caught in the small clearance as bars cross each other, and the material moves generally radially outward through the grooves between the bars. In this manner, material fed between the plates near the axis of rotation experiences thousands of pressure pulses at bar crossings as it progresses in a generally outward direction over and through successive bars and grooves, to be discharged at the circumferential periphery of the plates in a more refined state.
Refining of different kinds of feed materials, or achieving desired variations in output quality from the same kind of feed material, depending on the end usage, dictate a variety of possible design optimizations with respect to the variables of plate diameter, throughput rate, gap size, and refining intensity. These variables are well known to practitioners in the relevant field. In general, however, refiner plates have a substantially annular inner zone characterized by very coarse bars and grooves, where the feed material is reduced in size and given a radial component of movement, without substantial refining action. A second, intermediate annular zone receives the material from the first zone and performs a relatively coarse refining action. An outer annular zone of relatively fine, closely spaced bars and grooves provides a higher degree of refining. In refiners in which only one disc rotates the plate through which the material is fed into the gap, is generally referred to as the feed end plate. This plate has the relatively coarse, breaker bars defining the feed zone. The stationary plate, commonly referred to as the control end plate, usually does not have the coarse breaker bars, and does not rotate (although it may be adjusted axially to control the gap width).
In conventional refiner plates, the transition between zones, generally appears circular. More specifically, the plates are usually formed by attaching a plurality of segments side-by-side onto the disc surface, with the zone transitions being symmetric on either side of a radially extending central axis on each segment. Viewed differently, a line connecting the inlets of successive grooves in a given zone, when extended laterally to the sides of a plate segment, fall symmetrically on either side of the segment central axis. Usually, such line follows the arc of a circle, or two lines intersect at the axis, forming the same angle relative to the axis.
Despite the apparent operational simplicity of this conventional inlet configuration, inspection by the present inventor of plates after refiner operation, indicates local areas of concentrated, excessive wear along the imaginary line connecting the inlets for a particular refining zone. Such excessive localized wear unnecessarily limits the useful lifetime of the plates. The present inventor believes such wear arises from localized restriction, or pinching, of material flow between zones. These restrictions arise not so much from the inherent flow areas as viewed on an individual plate or segment, but rather from the localized restricting effect which occurs when one plate rotates with a very close gap relative to a confronting plate.